A circuit may be used to determine when a voltage signal transitions across a threshold level. For example, a microprocessor or a Very Large-Scale Integration (VLSI) circuit may need to determine when a processor voltage signal reaches an acceptable voltage level.
Traditionally, such a determination is made by a circuit that uses a stable voltage reference signal. FIG. 1 illustrates traditional relationships 100 between a processor voltage signal, a reference voltage signal, a slightly scaled processor voltage signal, and a power indication signal at different temperatures.
Consider first a reference voltage signal 110 generated at 100xc2x0 Centigrade (C). Note that the reference voltage signal 110 initially increases along with the processor voltage signal (Vcc) That is, the reference voltage signal 100 is about 400 millivolts (mV) when Vcc is 400 mV. Above that Vcc, the reference voltage signal 110 begins to stabilize. That is, the rate of increase of the reference voltage signal 110 begins to decrease (as compared to Vcc) when Vcc reaches approximately 600 millivolts (mV). Traditionally, only a limited number of stable reference voltage values can be produced by such a circuit (e.g., based on diode thresholds, silicon band gap voltages, and/or transistor thresholds associated with the circuit). In order to generate other reference voltage values, scaling circuits may be used.
A slightly scaled processor voltage signal 120 at 100xc2x0 C. is generated by scaling down Vcc (e.g., with resistors). As a result, the slightly scaled processor voltage signal 120 rises at a slightly slower rate as compared to Vcc. For example, the slightly scaled processor voltage signal 120 illustrated in FIG. 1 reaches approximately 1.0 Volt (V) when Vcc is 1.2 V.
A power indication signal 130 is then generated when the slightly scaled processor voltage signal 120 transitions past the reference voltage signal 110. The power indication signal 130 may indicate, for example, that Vcc has now reached an acceptable voltage level for a processor. The point (e.g., the Vcc) at which the power indication signal 130 is generated is determined by the transfer curve of the circuit. This point may be modified to a desired level by, for example, adjusting the resistance used to scale down the processor voltage.
There are several disadvantages, however, with the traditional methods of generating a power indication for a processor. For example, consider a reference voltage signal 112 that is generated when the temperature of the circuit is 0xc2x0 C. Note that this reference voltage signal 112 levels off at a higher value as compared to the reference voltage signal 110 at 100xc2x0 C. Also note that the slightly scaled processor voltage signal 122 at 0xc2x0 C. does not significantly change as compared to the signal 120 at 100xc2x0 C. As a result, the power indication signal 132 is not generated until a higher Vcc is reached (e.g., the power indication signal 132 at 0xc2x0 C. occurs approximately 200 mV after the power indication signal 130 at 100xc2x0 C.). This temperature sensitivity is undesirable because the pre-determined acceptable voltage level for the processor has not actually changed.
Moreover, because the difference between the reference voltage signal and the slightly scaled processor voltage signal is small, the circuit will be sensitive to voltage noise. For example, FIG. 2 illustrates traditional relationships 200 between Vcc, a reference voltage signal 210, a slightly scaled processor voltage signal 220, and a power indication signal 230 when 200 mV of Alternating Current (AC) noise is introduced to a traditional power indication circuit. Note that the power indication signal 230 is generated multiple times because the slightly scaled processor voltage signal 220 crosses the reference voltage signal 210 many times. This result is also undesirable because no clear indication of an acceptable voltage level is provided.